Superconductivity in materials allows a large magnetic field to be produced in a circuit containing a superconducting coil by the use of a relatively small source of magnetic field through a magnetic flux pumping action which requires no electrical connection between the source field and the recipient field. Disclosures about superconducting magnetic flux pumps have been made in the following references:
1. "Flux Pumps and Superconducting Solenoids", Van Beelen et al., Physica 31, 413 (1965). PA1 2. "The Principle and Performance of a Superconducting Dynamo", Suchtelen, et al., Cryogenics, 256 (1965).
The concept of superconducting magnetic flux pumps gives rise to the possibility of energizing the rotating field coil of an AC generator without the use of brushes, slip-rings or contacts of any kind. This concept therefore avoids the interface problems of a superconducting-to-normal transition for a superconducting field coil. It also avoids the problem of excessive evaporation of liquid helium caused by high current input leads. It also presents a means for the rapid de-excitation of the field coil to prevent its quench (loss of superconductivity) in the event of a serious fault. The latter advantages would also accrue to the superconducting field coil of a DC generator and more generally to any superconducting electromagnet.
Magnetic flux pumps which use the property of a superconducting circuit to trap magnetic flux have been known for well over ten years. This invention uses this property by simultaneously and sequentially introducing magnetic flux through gates embodying a modular design. The sequential excitation is in synchronism with the magnetic source field. Conventional flux pumps generally are constructed with a single gate, usually a thin superconducting foil in which a small transient area is caused to conduct current in a normal fashion during the flux trapping or pumping process. The gate may also be made in forms other than thin foils, such as an assembly of parallel wires, as long as the gate's critical field is relatively less than other parts of the circuit.
The idea of using a fluid pumping action to energize a superconducting field coil has previously been proposed although there has been no completely satisfactory solution to this problem before the conception of the present invention. The main problem associated with conventional magnetic flux pumps is the fact that, as a proper electrical current is induced in one portion of a superconducting circuit, electrical currents will be induced in other portions and these latter currents (countercurrents) flow in the opposite direction to that which is desired. This results in inefficiencies and, because of the "persistence" of supercurrents, will require a relatively longer time for the complete excitation or de-excitation of the magnetic field in a field coil. This is due to the fact that the currents flowing in the opposite direction must be overcome and a longer time is required for this purpose. As a result of these shortcomings, a need has arisen for an improved flux pump which is highly efficient in its pumping action; yet is relatively simple in construction.
Another problem with prior art magnetic flux pumps relates to their non-optimal gatig action which also limits the rate of excitation or de-excitation of the field coil. The present invention has the advantageous feature of multiple sequential introduction of flux in a given module to overcome this problem.